The invention relates to a fuel injection unit for each cylinder of a diesel engine, which consists of a piston pump receiving a quantity of fuel which can be regulated by means of a slanted-edge control and of an injection nozzle provided with a spring-loaded nozzle needle, the injection nozzle being arranged in a common housing with the piston pump, and wherein the pump cylinder includes at least one fuel borehole, which communicates with a pump suction chamber and wherein the pump piston is, in turn, provided with a front control edge and rear control edge, the pump cylinder also including a fastening flange which, by means of a tension sleeve or the like, can be held under tension against a pump cylinder cover between the fuel borehole and the pump-cylinder end facing the cover, the piston pump, during a stroke thereof, passing the fuel borehole.
In the case of known injection units of this type, the regulation of the quantity of fuel to be injected, takes place through rotation of the pump piston. When, during the stroke of the piston, its front control edge passes over the fuel borehole in the pump cylinder, the inside area of the pump cylinder is closed-off and the fuel injection begins. It lasts up to the time when the rear control edge has reached, and releases the fuel borehole. Through a longitudinal groove or the like in the pump piston, the fuel enclosed within the interior of the cylinder can then escape into the pump suction chamber during the remaining stroke of the pump piston, so that the fuel pressure steeply decreases, and the injection nozzle is again closed by the spring-loaded nozzle needle.
For the attainment of as favorable a combustion course as possible, in particular in the case of modern diesel engines with direct fuel injection, high injection pressures of more than 1000 bars are strived for, which can only be attained in the case of a narrow slit being formed between the pump piston and the pump cylinder having a maximum diversion of 1.5 .mu.m. For the sealing of the pump cylinder with respect to the pump cylinder lid, the high injection pressures require a high initial pressure or prestress acting on the fastening flange, but therein such high injection pressures may in turn, cause a constriction of the working surface of the pump cylinder, which again imposes the utilization of an undesirably great play or clearance of the pump piston. As a result of this unduly large play, an irregular fuel feed, which depends on the stroke of the pump piston and on the engine revolutions can occur, which in turn adversely affects the accuracy of the injection process. Moreover, the sudden steep pressure increase in the pump cylinder, after closure of the fuel borehole, covers a risk of pressure fluctuations, which require a still higher initial pressure and lead to further inaccuracies at the beginning of the injection, as well as during the injection or at the end of the injection. These disadvantageous effects show up in particular in diesel engines of vehicles which are operated at a broad range of the number of revolutions, with a high peak in the number of revolutions, and, in the case of fuel injection units of the initially cited type, are of great importance, since after installation in the diesel engines, these fuel injection units can no longer be re-adjusted.